High frequency fluid driven drill hammer percussion drilling in hard formations

ABSTRACT

A fluid pressure driven, high frequency percussion hammer for drilling in hard formations is presented. The hammer piston ( 20 ) of the percussion hammer has a relatively large and longitudinally extending bore ( 41 ) that provides minimal flow resistance for a drilling fluid flowing through the bore ( 41 ) during the return stroke of the hammer piston ( 20 ). The bore ( 41 ) is closeable in the upstream direction by a valve plug ( 23 ) that follows the hammer piston ( 20 ) during the stroke. The valve plug ( 23 ) is controlled by a relatively long and slender valve stem ( 49 ) that is mechanically able to stop the valve plug ( 23 ) by approximately 75% of the full stroke length of the hammer piston ( 20 ) and separates the plug ( 23 ) from a seat ring ( 40 ). Thus the bore ( 41 ) opens up such that the bore fluid can flow there trough, and the inherent tension spring properties of the valve stem ( 49 ) returns the valve plug ( 23 ) so rapid that it will be good through flow during return of the hammer piston ( 20 ).

The present invention relates to a fluid pressure driven, high frequencypercussion hammer for drilling in hard formations, which percussionhammer comprises a housing, which in one end thereof is provided with adrill bit designed to act directly on the hard formation, whichpercussion hammer further comprises a hammer piston moveably received insaid housing and acts on the drill bit, which hammer piston has alongitudinally extending bore having predetermined flow capacity, andthe bore being closeable in the upstream direction by a valve plug thatpartly follows the hammer piston during its stroke.

Hydraulically driven percussion hammers for drilling in rock have beenin commercial use for more that 30 years. These are used with jointabledrill rods where the drilling depth is restricted by the fact that thepercussion energy fades through the joints, in addition to the fact thatthe weight of the drill rod becomes too heavy such that little energyfinally reaches the drill bit.

Downhole hammer drills, i.e. hammer drills installed right above thedrill bit, is much more effective and are used in large extent fordrilling of wells down to 2-300 meter depth. These are driven bycompressed air and have pressures up to approximately 22 bars, whichthen restricting the drilling depth to approximately 20 meters if wateringress into the well exists. High pressure water driven hammer drillshave been commercial available more than 10 years now, but these arelimited in dimension, which means up to about 130 mm hole diameter. Inaddition, they are known to have limited life time and are sensitive forimpurities in the water. They are used in large extent in the miningindustries since they are drilling very efficiently and drill verystraight bores. They are used in a limited extent for vertical welldrilling down to 1000-1500 meters depth, and then without anydirectional control.

It is desired to manufacture downhole drill fluid driven hammer drillswhich can be used together with directional control equipment, whichhave high efficiency, can be used with water as drill fluid and can alsobe used with water based drill fluid having additives, and havingeconomical lifetime. It is expected great usage both for deepwaterdrilling for geothermic energy and for hard accessible oil and gasresources.

In percussion drilling, drill bits are used having inserted hard metallugs, so called “indenters”. These are made of tungsten carbide and aretypically from 8 to 14 mm in diameter and have spherical or conical end.Ideally viewed, each indenter should strike with optimal percussionenergy related to the hardness and the compressive strength of the rock,such that a small crater or pit is made in the rock. The drill bit isrotated such that next blow, ideally viewed, forms a new crater havingconnection to the previous one. The drilling diameter and the geometrydetermine the number of indenters.

Optimal percussion energy is determined by the compressive strength ofthe rock, it can be drilled in rock having compressive strength over 300MPa. The supply of percussion energy beyond the optimal amount, is lostenergy since it is not used to destroy the rock, only propagates aswaves of energy. Too little percussion energy does not make craters atall. When percussion energy per indenter is known and the number ofindenters is determined, then the optimal percussion energy for thedrill bit is given. The pull, or drilling rate, (ROP—rate ofpenetration) can then be increased by just increasing the percussionfrequency.

The amount of drilling fluid pumped is determined by minimum necessaryreturn rate (annular velocity) within the annulus between the drillstring and the well bore wall. This should at least be over 1 m/s,preferably 2 m/s, such that the drilled out material, the cuttings, willbe transported to the surface. The harder and brittle the rock is, andthe higher percussion frequency one is able to provide, the finer thecuttings become, and the slower return rate or speed can be accepted.Hard rock and high frequency will produce cuttings that appear as dustor fine sand.

The hydraulic effect applied to the hammer drill is determined by thepressure drop multiplied with pumped quantity per time unit.

The percussion energy per blow multiplied with the frequency providesthe effect. If we look into an imaginary example where drilling intogranite having 260 MPa compressive strength and drilling diameter of 190mm is performed, water is pumped by 750 l/min (12.5 liters/second) fromthe surface. It is calculated that approximately 900 J is optimalpercussion energy.

With reference to known data for corresponding drilling, but withsmaller diameters, a drilling rate (ROP) of 22 m/h (meters per hour)with a percussion frequency of 60 Hz, can be expected. It is hereassumed to increase the percussion frequency to 95 Hz, consequently ROPthen become 35 m/h. Required net effect on the drill bit then becomes:0.9 kJ×95=86 kW. We assume the present hammer construction to have amechanical-hydraulic efficiency of 0.89, which then provides 7.7 MParequired pressure drop over the hammer.

This hammer drill will then drill 60% quicker and by 60% less energyconsumption than known available water propelled hammer drills.

This is achieved by a percussion hammer of the introductory said kind,which hammer is distinguished in that the valve plug is controlled by anassociated valve stem slideably received in a valve stem sleeve, saidvalve stem comprises stopping means able to stop the valve plug by apredetermined percentage of the full stroke length of the hammer pistonand separates the valve plug from a seat seal on the hammer piston, saidbore thus being opened and allows the bore fluid to flow through thebore.

Preferably the stopping means comprises a stop plate at the upstream endof the valve stem, and a cooperating internal stop surface in the valvestem sleeve.

In one embodiment the predetermined percentage of the full stroke lengthof the hammer piston can be in the order of magnitude 75%.

Conveniently, it is the inherent tension spring properties of the valvestem that returns the valve plug, which valve stem being long andslender.

Preferably, the percussion hammer can further be provided with an inletvalve assembly, which is not opening for operation of the hammer pistonuntil the pressure is build up to approximately 95% of full workingpressure, which inlet valve assembly being adapted to close off a mainbarrel, and a side barrel within the hammer housing can pressurize anannulus between the hammer piston and the housing elevating the hammerpiston to seal against the valve plug.

Conveniently, the hammer piston and the valve assembly can be returnedby recoil, where both the hammer piston and the valve assembly areprovided with hydraulic dampening controlling the retardation of thereturn stroke until stop.

Conveniently, the hydraulic dampening takes place with an annular pistonwhich is forced into a corresponding annular cylinder with controllableclearances, and thus restricts or chokes the evacuation of the trappedfluid.

Further, an opening can be arranged in the top of the valve stem sleeve,into which opening the stop plate of the valve stem is able to enter,and the radial portions of the stop plate can seal against the internalside of the opening with relatively narrow radial clearance.

Further, an annular backup valve can be arranged in a ring grooveunderneath the opening, which backup valve being able to open andreplenish fluid through bores in the valve stem sleeve.

The percussion hammer housing can be divided into an inlet valvehousing, a valve housing and a hammer housing.

The hammer drill construction according to the present invention is ofthe type labeled “Direct Acting Hammer”, i.e. that the hammer piston hasa closing valve thereon, which valve in closed position enables thepressure to propel the piston forward, and in open position enables thehammer piston to be subjected to recoil. The previous variant of ahydraulic hammer has a valve system that by means of pressure propelsthe hammer piston both ways. This provides poor efficiency, but moreprecise control of the piston.

The key to good efficiency and high percussion frequency, is in thevalve construction. The valve needs to operate with high frequency andhave well through flow characteristics in open position.

With great advantage, the hammer drill construction can also be used assurface mounted hydraulically driven hammer for drilling with drillrods, but it is the use as a downhole hammer drill that will bedescribed in detail here.

Other and further objects, features and advantages will appear from thefollowing description of preferred embodiments of the invention, whichis given for the purpose of description, and given in context with theappended drawings where:

FIG. 1 shows in schematic view a typical hydraulic hammer drillaccording to the invention,

FIG. 2A shows an elevational view of a downhole hammer drill with drillbit,

FIG. 2B shows the hammer drill of FIG. 2A turned about 90°,

FIG. 2C shows a view in the direction of the arrows A-A in FIG. 2A,

FIG. 2D shows a view in the direction of the arrows B-B in FIG. 2A,

FIG. 3A shows a longitudinal sectional view of the hammer drill shown inFIG. 2A where the internal main parts are shown,

FIG. 3B shows a transversal cross sectional view along the line A-A inFIG. 3A,

FIG. 3C shows a transversal cross sectional view along the line B-B inFIG. 3A,

FIG. 3D shows a transversal cross sectional view along the line C-C inFIG. 3A,

FIG. 3E shows a transversal cross sectional view along the line D-D inFIG. 3A,

FIG. 3F shows a two times enlarged, encircled detail view H in FIG. 3A,

FIG. 3G shows a two times enlarged, encircled detail view H in FIG. 3A,

FIG. 3H shows a five times enlarged, encircled detail view F in FIG. 3A,

FIG. 3I shows a five times enlarged, encircled detail view G in FIG. 3A,

FIG. 4A shows correspondingly to that shown in FIG. 3A, but at the endof an acceleration phase,

FIG. 4B shows an elevational view of the valve assembly shown in sectionin FIG. 4A,

FIG. 4C shows a transversal cross sectional view along the line B-B inFIG. 4A,

FIG. 4D shows a five times enlarged, encircled detail view A in FIG. 4A,

FIG. 4E shows a five times enlarged, encircled detail view C in FIG. 4A,

FIG. 5A shows correspondingly to that shown in FIGS. 3A and 4A, but inthat moment when the hammer piston strikes against the impact surface inthe drill bit,

FIG. 5B shows a five times enlarged, encircled detail view A in FIG. 5A,

FIG. 5C shows a four times enlarged, encircled detail view B in FIG. 5A,

FIG. 6A shows correspondingly to that shown in FIGS. 3A, 4A and 5A, butwhen the hammer piston is in full return,

FIG. 6B shows a section along the line E-E in FIG. 6C,

FIG. 6C shows a five times enlarged, encircled detail view A in FIG. 6A,

FIG. 6C′ shows a 20 times enlarged, encircled detail view D in FIG. 6C,

FIG. 6D shows a 20 times enlarged, encircled detail view C in FIG. 6E,

FIG. 6E shows a four times enlarged, encircled detail view B in FIG. 6A,

FIG. 7A shows correspondingly to that shown in FIGS. 3A, 4A, 5A and 6A,but when the hammer piston is in the final part of the return,

FIG. 7B shows a 20 times enlarged, encircled detail view B in FIG. 7C,

FIG. 7C shows a four times enlarged, encircled detail view A in FIG. 7A,

FIG. 8 shows curves that illustrates the working cycle of the hammerpiston and the valve,

FIG. 9A shows the curve that illustrates the abrupt closingcharacteristic of the valve relative to pressure drop, and

FIG. 9B illustrates flow and pressure drop over the gradually closingvalve.

FIG. 1 shows a typical hydraulic hammer drill for attachment on top ofjointable drill rods where the hammer mechanism is located internal of ahousing 1 constructed by several house sections, where a rotary motor 2rotates a drill rod via a transmission 3 rotating an axle having athreaded portion 4 to be screwed to the drill rod and a drill bit (notshown). The hammer machine is normally equipped with a fixation plate 5for attachment to a feeding apparatus on a drill rig (not shown). Supplyof hydraulic drive fluid takes place via pipes and a coupling 6 andhydraulic return via pipes with a coupling 7.

FIGS. 2A and 2B show a downhole hammer drill with drill bit. These willbe used in the following description. The illustrated housing 1 has afirst house section 8 that receives what later on will be described asthe inlet valve, while a second house section 9 contains a valve, athird house section 10 contains a hammer piston and the reference number11 denotes the drill bit. Drill fluid is pumped in through an opening ormain run 12, and a threaded portion 13 connects the hammer to the drillstring (not shown). A flat portion 14 is provided for use of a torquewrench to screw the hammer to/from the drill string. A drain hole 15 isrequired for the function of the later on explained inlet valve, outlethole 16 is present for return of the drill fluid in the annulus betweenthe drill hole wall and the hammer drill housing (not shown) back to thesurface. Hard metal lugs 17 are those elements that crush the rock beingdrilled. FIG. 2C shows a view in the direction of the arrows A-A in FIG.2A, and FIG. 2D shows a view seen towards the drill bit 11 in thedirection of the arrows B-B in FIG. 2A.

FIG. 3A shows a longitudinal section of the hammer drill where theinternal main parts are: an inlet valve assembly 18, a valve assembly 19and a hammer piston 20. The drilling fluid is pumped in through theinlet 12, passes the inlet valve 18 in open position through bores 21shown on section A-A in FIG. 3B, further through bores 22 in section B-Bin FIG. 3C to a valve plug 23 that is shown in closed position insection C-C in FIG. 3D against the hammer piston 20 and drives thepiston to abutment against the bottom portion 24 of the drill bit.Section D-D in FIG. 3E shows a longitudinally extending spline portion25 in the drill bit 11 and the lowermost part of the hammer housing 10that transfer the torque at the same time as the drill bit 11 can moveaxially within accepted clearances determined by a locking ringmechanism 26. This because by blows of the hammer piston 20 against thedrill bit 11, it is only the mass or weight of this that is displaced inconcert with penetration of the hard metal lugs 17 into the rock. Thisis to obtain that as much as possible of the percussion energy shall betransferred to the crushing of the rock and as little as possible to belost to mass displacement of the relatively light drill bit 11.

The detailed section in FIG. 3F showing the inlet valve 18 in closedposition is taken from H in FIG. 3A. When the hammer function is to beinitiated, the pumping operation of the drill fluid in the inlet 12 iscommenced. A side, or branch off, bore 27 through the wall of the valvehouse 8 has hydraulic communication with a pilot bore 28 in the mountingplate 29 of the inlet valve 18. The mounting plate 29 is stationary inthe valve house 8 and contains a pilot valve 30 that is retained in openposition by a spring 31. The drill fluid flows freely to a first pilotchamber above a first pilot piston 32, the diameter and area of whichare larger than the area of the inlet 12. During pressure buildup, alimited moveable valve plug 33 will be forced to closure against a valveseat 34 in the housing 8. Under pressure buildup against closed inletvalve 18, an annulus 35 between the housing 10 and the hammer piston 20is pressurized through the side bore 27, which via longitudinallyextending bores 36 in the valve housing 9 feed an inlet 37, see detailedview F.

The detailed sections in FIG. 3H and FIG. 3I are taken from F and G inFIG. 3A and show the abutment of the hammer piston 20 against the innerwall of the hammer housings 9, 10. The diameter of a piston 38 issomewhat larger than the diameter of a second piston 39. By the use ofthe hammer drill to drill vertically downwards, the hammer piston 20will in unpressurized condition, due to the gravity, obviously creeptowards the strike or impact surface 24 in the drill bit 11. In thiscondition there will be clearance between the valve plug 23 and its seat40 (see detailed view F) in the hammer piston 20. Accordingly the drillfluid will flow freely through the valve at the plug 23, through a bore41 in the hammer piston 20 and the bores 16 (see FIG. 2A), and thereforetoo little pressure buildup takes place to start the hammer.

The arrangement shown in detailed section in FIG. 3F, having closedinlet valve 18 and pressure buildup in the annulus 35, elevates thehammer piston 20 to seal against the valve plug 23. Due to the requiredclearance between the surface of the piston 38 and the inner wall of thehousing 9, drilling fluid leaks out in the space above the valve plug 23through lubrication channels 42 and a bore 43 such as an arrow shows indetailed view F. In order to prevent that this leakage volume shallprovide pressure buildup in the space above the valve plug 23, this isdrained through a bore 44 in the valve mounting plate 29 and an opening45 that the pilot valve 30 in this position allows, and further outthrough the drain hole 15. When the pressure has increased to over 90%of the working pressure the hammer is designed for, the piston force ina second pilot chamber 46 exceeds the closing force of the spring 31 andthe pilot valve 30 shifts position such as illustrated in FIG. 3G.

The first pilot chamber above the pilot piston 32 is drained and theinlet valve 18 opens up. At the same time the opening 45 is closed suchthat drainage through the bore 44 is shut off so that pressure is notlost through this bore in operating mode. The pressure in the chamberabove the hammer piston 20 and the closed valve plug 23 results in startof the working cycle with instant full effect. The arrangement with abackup valve 47 and a nozzle 48 is provided to obtain a reduced drainagetime of the second pilot chamber 46 for thereby achieve relatively slowclosure of the inlet valve 18. This to obtain that the inlet valve 18remains fully open and is not to make disturbances during a working modesince the pressure then fluctuates with the percussion frequency.

FIG. 4A shows the hammer drill at the end of an accelerating phase. Thehammer piston 20 has at this moment arrived at max velocity, typicallyabout 6 m/s. This is a result of available pressure, as an example herejust below 8 MPa, the hydraulic area of the hammer piston, here forexample with a diameter of 130 mm, and the weight of the hammer piston,here for example 49 kg. The valve plug 23 is kept closed against theseat opening of the hammer piston since the hydraulic area of the valveplug 23, here for example with a diameter of 95 mm, is a bit larger,about 4%, than the annular area of the hammer piston shown in sectionB-B in FIG. 4C as 23 and 24 respectively. At this moment the hammerpiston has covered about 75% of its full stroke, about 9 mm. Theclearance between the hammer piston 20 and the strike surface 24 of thedrill bit is about 3 mm, shown in enlarged detailed view C in FIG. 4E.

A moveable valve stem 49 having a stop plate 50 now lands on theabutment surface of a stationary valve stem sleeve 51 in the housing 9and stops the valve stem 49 from further motion, as shown in enlargeddetailed view A in FIG. 4D, after which the valve plug 23 is separatedfrom the seat 40 in the hammer piston 20 and thereby being opened. Themoveable valve assembly 23, 49, 50 is shown in elevational view in FIG.4B.

The kinetic energy of the valve plugs 23 momentum will by the abruptstop thereof marginally elongate the relatively long and slender valvestem 49, and thereby transform to a relatively large spring force thatvery quick accelerates the valve in return. The marginal elongation ofthe valve stem 49, here as an example calculated to be about 0.8 mm,needs to be lower than the utilization rate of the material, whichmaterial in this case is high tensile spring steel. The mass of thevalve plug 23 should be as small as possible, here as an example made ofaluminum, combined with the length, the diameter and the properties ofthe material of the valve stem 49, determines the natural frequency ofthe valve assembly.

For practical usages, this should be minimum 8-10 times the frequency itis to be used for. The natural frequency is determined by the formulas:

${fn} = {{\frac{1}{2\pi}\sqrt{\frac{k}{M}}\mspace{14mu}{where}\mspace{14mu} k} = \frac{F}{\sigma}}$

The mass and the spring constant have most significance. The naturalfrequency for the shown construction is about 1100-1200 Hz and thereforeusable for a working frequency over 100 Hz.

The shown construction has in this example a recoil velocity that is 93%of the impact or strike velocity.

FIG. 5A shows the position and the moment for when the hammer piston 20strikes against the strike or abutment surface 24 within the drill bit11. The valve plug 23 including the stem 49 and the stop plate 50 are infull return speed, see detailed view A in FIG. 5B, such that relativelyfast a large opening between the valve plug 23 and the valve seat 40 onthe hammer piston 20 is created, such that drilling fluid now flows byrelatively small resistance through the longitudinal bore 41 in thehammer piston 20, see detailed view B in FIG. 5C.

The kinetic energy of the hammer pistons 20 momentum is partlytransformed into a spring force in the hammer piston 20, since thepiston is somewhat compressed during the impact. When the energy wavefrom the impact has migrated through the hammer piston 20 to theopposite end and back, the hammer piston 20 accelerates in return. Thereturn velocity here at the start is calculated to be about 3.2 m/s,about 53% of the strike or impact velocity, this because a portion ofthe energy has been used for mass displacement of the drill bit 11,while the rest has been used to depress the indenters into the rock.

FIG. 6A shows that moment when the hammer piston 20 is in its fullreturn speed. The valve plug 23 has at this point of time almostreturned to the end stop where the detailed view A in FIG. 6C shows thestem 49 including the stop plate 50 entering an opening 52 in the top ofthe valve stem sleeve 51.

The detailed view D in FIG. 6C′ shows how the radial portion of the stopplate 50 seals, with relatively narrow radial clearance, against theinternal side of the opening 52. A small negative pressure is created inthe chamber underneath the stop plate 50 when the stop plate 50 movesthe last 2 mm until stop. An annular backup valve 58 opens andreplenishes liquid through the bore 59. The confined or trapped volumeunder the stop plate 50 prevents that the valve plug 23 performs arecoil motion and remains in position until next cycle starts.

The backup valve 58 of the type “annular backup valve”, which in thisembodiment is an annular leaf spring, is chosen since this has littlemass and relatively large spring force and accordingly is able to workwith high frequency.

The detailed view B in FIG. 6E shows the relatively large openingbetween the valve plug 23 and the valve seat 40 in the hammer piston 20,in order that the flow of drilling fluid there through takes place witha minimum of resistance. The underside of the valve stem sleeve 51 isformed as an annular cylinder pit 53 shown in detailed view C in FIG.6D. The top of the valve plug 23 is formed as an annular piston 54,which by relatively narrow clearances fits into the annular cylinder pit53. The confined fluid volume is, as the valve returns all the way tothe end stop, evacuated in a controlled way through the radialclearances between the annular piston 54 and the annular cylinder 53plus an evacuation hole 55. This controlled evacuation acts as adampening force and stops the return of the valve in such a way that thevalve does not perform recoil motions. The same type of dampeningarrangement is present on the hammer piston 20. On the detailed view Bis an annular piston 56 shown on top of the hammer piston 20, inaddition to an annular cylinder groove 57 in the lower part of the valvehousing 9.

FIG. 7A shows the last part of the return of the hammer piston 20. Thetermination of the return stroke is dampened in a controlled way untilfull stop at the same time as the valve seat 40 meets the valve plug 23,shown in detailed view A in FIG. 7C. The detailed view B in FIG. 7Billustrates how the confined or trapped fluid volume within the annularcylinder pit 57 is displaced through the radial clearances between theannular piston 56 and a drain hole 60.

The gap between the valve seat 40 and the valve plug 23 needs not to beclosed completely in order that the pressure to build up and a new cyclestarts.

Calculations show that with an opening of 0.5 mm the pressure drop isapproximately the same as the working pressure. This results in that thesurface pressure on the contact surface between the valve plug 23 andthe seat 40 becomes small and the components can experience long lifetime.

FIG. 8 shows curves that illustrate the working cycle of the hammerpiston 20 and the valve. Curve A shows the velocity course and curve Bthe position course through a working cycle. For both curves thehorizontal axis is the time axis, divided into micro seconds.

The vertical axis for curve A shows the velocity in m/s, strokedirection against the drill bit 11 as + upwards and − downwards, herethe return velocity.

The vertical axis for the curve B shows distance in mm from the startposition. The curve section 61 shows the acceleration phase, where thepoint 62 is the moment when the valve is stopped and the return thereofis initiated. The point 63 is the impact of the hammer piston 20 againstthe drill bit 11.

The curve section 64 is the displacement of the drill bit 11 by progressinto the rock, 65 is the acceleration of the recoil, 66 is the returnvelocity without dampening and 67 is the return velocity with dampening.The curve section 68 is the recoil acceleration for the valve, 69 is thereturn velocity for the valve without dampening and 70 is the slow downdampening phase for the return of the valve.

FIG. 9A shows a curve 71 that illustrates the abrupt closingcharacteristics for the valve with regard to the pressure drop andopening between the valve plug 23 and the seat 40 in the hammer piston.This situation is shown in FIG. 9B. The horizontal axis is the openinggap in mm and the vertical axis the designed pressure drop in bar atnominal rate of pumped drilling fluid, which, as an example here, is12.5 l/sec. As shown, the closing gap needs to get under 1.5 mm before asubstantial pressure resistance is received.

The invention claimed is:
 1. A fluid pressure driven high frequencypercussion hammer for drilling in hard formations, the percussion hammercomprising: a housing which in one end thereof is provided with a drillbit designed to act directly on the hard formation; a hammer pistonmoveably received in said housing and that acts on the drill bit, thehammer piston having formed therein a longitudinally extending borehaving a predetermined flow capacity, the longitudinally extending borebeing closeable in an upstream direction by a valve plug that partlyfollows the hammer piston during its stroke; a valve stem operativelycoupled to the valve plug and slideably received in a valve stem sleeve,said valve stem comprising: a stop plate disposed at an upstream end ofthe valve stem, the stop plate engaging the valve stem sleeve to: 1)stop the valve plug at a predetermined percentage of a full strokelength of the hammer piston thereby opening the bore; and 2) elasticallystrain the valve stem such that said elastically strained valve stem isrecoiled back to an uppermost position of the valve stem in the valvestem sleeve by non-helical tension spring properties of the valve stem;and wherein said hammer piston is mechanically compressed when strikingthe drill bit so that said mechanically compressed hammer piston isrecoiled back towards the uppermost position of the hammer piston in thehousing due to pressure tension properties of the hammer piston.
 2. Thepercussion hammer according to claim 1, wherein the stop plate isadapted to cooperate with an internal stop surface in the valve stemsleeve.
 3. The percussion hammer according to claim 1, wherein thepredetermined percentage of the full stroke length of the hammer pistonis in an order of magnitude 75%.
 4. The percussion hammer according toclaim 1, wherein said valve stem being long and slender.
 5. Thepercussion hammer according to claim 1, comprising an inlet valveassembly, which inlet valve assembly does not open for operation of thehammer piston until the pressure is built up to approximately 95% offull working pressure, said inlet valve assembly being adapted to closeoff a main barrel, and that a side barrel within the hammer housingpressurizes an annulus between the hammer piston and the housingelevating the hammer piston to seal against the valve plug.
 6. Thepercussion hammer according to claim 5, wherein the hammer piston andthe inlet valve assembly return by recoil, where both the hammer pistonand the inlet valve assembly are provided with hydraulic dampeningcontrolling retardation of a return stroke until stop.
 7. The percussionhammer according to claim 6, wherein the hydraulic dampening takes placeby an annular piston, which annular piston is forced into acorresponding annular cylinder having controllable clearances, and thusrestricts or chokes evacuation of trapped fluid.
 8. The percussionhammer according to claim 1, wherein an opening is arranged in a top ofthe valve stem sleeve, into which opening a stop plate of the valve stemis able to enter, radial portions of the stop plate seals against aninternal side of the opening with relatively narrow radial clearance. 9.The percussion hammer according to claim 8, wherein an annular backupvalve is arranged in a ring groove underneath the opening, wherein thebackup valve is able to open and replenish fluid through bores in thevalve stem sleeve.
 10. The percussion hammer according to claim 1,wherein the percussion hammer housing is divided into an inlet valvehousing, a valve housing and a hammer housing.